The disclosures herein relate generally to computer systems and more particularly to a perforated gasket for providing an electromagnetic interference seal for a computer chassis enclosure.
There is a widespread problem of trying to close, or fill, gaps in chassis enclosures, especially removable-cover seams. The ability to close these gaps is essential in order to pass the FCC's electromagnetic interference (EMI) requirement and well as electrostatic discharge susceptibility.
Conductive foam gaskets have proven to be the most robust and cost effective solution to providing an EMI seal. However, traditional foam gaskets pose a number of problems.
The bigger/taller the gasket profile, or cross-section, the greater it's range of compression. However, the problem is further complicated by cover and chassis geometry. Firstly, a foam gasket is selected that, theoretically, gives the required range of compression, given the theoretical tolerances (and theoretical forces). But if this gasket generates forces, which either deform the covers so subsequent gaps are created, or the net forces are too high for ergonomic requirements, then a larger gasket is selected that generates less force for a given range of compression. Most often, both tolerances and actual forces contribute to the problem, invariably due to design changes and variance in the parts throughout the product design/development cycles. However the chassis design must be revised to accommodate the larger volume gasket, if possible. Often the space is simply not available. In thin rack servers this is the case because the residual height of the gasket after maximum allowable compression must be accommodated and that space is not available. When engineers initially “pad” their designs with excessive gasket volumes, the computer designs as a whole will be subsequently degraded from lost volume or other geometric/space conflicts. Whole programs maybe abandoned or disabled due to this practice. Therefore, any solution that incrementally reduces the compressive forces relative to range of compression for a gasket helps tremendously.
Two other solutions are commonly used to solve the above problems; custom spring fingers and wire mesh gaskets. Custom spring fingers are far more expensive (if made from Beryllium Copper or Phosbronze) or not as resilient as foam core gaskets. Additionally, spring fingers are not as robust in terms of customer access as they can easily hang up on passing objects, getting permanently deformed or broken off. Wire mesh gaskets have an inherent problem with having to be sealed at their ends to prevent unraveling. This causes the ends to be too stiff, thereby countering the high compliance given by the middle sections. Also, there is much more difficulty in adhering them to the covers or chassis as there are no continuous surfaces to apply a contact adhesive. This lack of continuous contact surfaces also causes the wire mesh to be of less value in term of radio frequency (RF) attenuation or electrostatic discharge (ESD) conductivity.
Chassis designers face another general problem concerning gap closure; non-uniform distortion of covers. Parts deflections under load (aside from coil springs) produce various complex deflection curves. This deflection curve, all too often, causes covers to bow away from the chassis to the point where a gap develops along the seam. Even a miniscule gap of a few thousandths of an inch can cause the computer to fail EMI or ESD requirements.
An additional problem encountered is that a linear gasket provides a force/unit length proportional to the compression in the same unit length. In many cases, the compression is severely uneven over the length that the gasket is being used. For example, on a hinged door with a latch on the outside edge, there would be much more compression (and more force) toward the hinge and toward the latch than there would be in the center of the door. Using a standard gasket tends to deform such a door, and potentially does not provide enough force to electrically seal the door in the center. What is ideally needed is a gasket that provides a varying force-compression curve along its length. Again, in the case of a latched door, it would provide more force in the center, and less toward the hinge and latch, optimally providing a constant force per unit length while the door is closed and latched.
Therefore, what is needed is a gasket that provides EMI shielding and generates less force than a traditional gasket, and that has the ability to vary the force provided along the length of the gasket.